In situ burning has received considerable attention in recent years as a cleanup method for oil in both upland ecosystems and on water. In situ burning involves the ignition and burning of oil spilled on water, vegetation or soils. The use of burning to address large scale oil spills at sea dates back to the Torrey Canyon incident in 1967. The failure of that and similar efforts discouraged the use of in situ burning until interest was renewed by the successful burn during the Exxon Valdez incident in 1989. Since then laboratory tests, the Mobile AL burn tests in 1991-1994, the Alaska Emulsion Burn Experiments, and the Newfoundland Offshore Burn Experiment showed that when properly managed in situ burning is a rapid, effective and environmentally safe technology for removing large quantities of floating oil. As a result, many authorities now consider burning as a valuable and effective tool rather than a method of last resort in the event of a major spill.
The ability of burning to quickly remove spilled oil and prevent its spread to sensitive sites or larger areas is perhaps burning's greatest advantage as a response strategy. Because oil is destroyed rather than collected, burning is attractive where transportation and disposal options are limited. It may be the only viable alternative in many remote locations where mechanical, dispersant and no-cleanup options are more damaging to the environment. Burning merits' special consideration for remediation of wetland environments or other oil-contaminated sites where access is limited or at sites where other methods prove ineffective or excessively intrusive. Several studies suggest that burning of spilled oil on open water and in upland environments may be more effective and more environmentally benign than intrusive mechanical and chemical treatments.
The ability of oil slicks to sustain combustion on open water depends largely on the thickness of the oil film. Oils will ignite if they are at least 2-3 mm thick and will burn down to slicks 1-2 mm thick. As oil is burned, the slick thins to a point where sufficient heat is lost to the underlying water to lower temperatures below that required to sustain combustion. In addition, “boilover” may occur when underlying water layers reach boiling temperatures, quenching the burn. Because oil on open water rapidly spreads to equilibrium thickness that is frequently less than the 2-3 mm needed for sustained combustion, heat-resistant booms are sometimes employed to entrap oil and maintain adequate thickness for efficient burning. This technique is costly and cumbersome to deploy, and therefore its use has been restricted to spills in remote yet assessable areas on relatively calm waters. Timeliness is far more critical to the success of marine burns than to inland burns where spilled oil has been successfully burned months and even years after impact. Dispersion rapidly thins most oil slicks to a degree that will not maintain combustion without confining booms. Spilled oil can rapidly emulsify when left to float on water and emulsification is accelerated by wave action. Attempts to burn heavily emulsified oil have not been successful. Evaporative losses of hydrocarbons, or weathering, results in the loss of the more easily ignited and burned volatile and semi-volatile components. Burning of heavier oils has proven difficult.
Because of the tendency of spilled oil to disperse on water, containment is required not only to prevent spreading but also to concentrate oil so that slicks are of sufficient thickness to ignite and burn efficiently. Without the benefit of containment booms, burning can only be accomplished within the first few hours after a spill event because oil rapidly spreads to equilibrium thickness. This thickness ranges between 0.01 to 0.1 mm for light crudes and fuel oils and 0.05-0.5 mm for heavy crudes and oils. In recent years, fire-resistant booms have been developed to facilitate burning and more are under development. Ideally, booms for burning should not only be fire-resistant, but lightweight, sufficiently flexible to accommodate waves, and easily deployed.
During the early development of marine burning of oil, techniques for ignition of the slick were a principal focus, no doubt because of the difficulty encountered when attempting to ignite slicks. It is now well established that slick thickness and oil type and condition are the principal factors influencing ignition and continued burning. Many devices have been devised to supply sufficient heat to ignite contained slicks with sufficient thickness for burning. They may be as simple as a roll of toilet-paper or rag soaked in diesel fuel and tossed into the slick, or as sophisticated as the “heliotorch”, a device suspended from a helicopter that drops burning packets of gelled gasoline. Similar devices are used by forestry companies and agencies to create back fires and therefore are available in most localities.
In the early 1970's, prior art attempts were made to design non-combustible silicate-based foams to be applied to oil slicks as absorbent material to support burning. See U.S. Pat. No. 3,698,850 to Sparlin; U.S. Pat. No. 3,843,306 to Whittington et al.; and U.S. Pat. No. 3,696,051 to McGuire et al., 1972). It is believed that no field evaluation of these materials was ever performed. Failure to test these materials may have been the result of unfortunate timing since at the time these patents were issued, burning oil at sea was considered a problem, not a solution. Also, because these materials were not biodegradable, they posed a potential hazard to sea life. Additionally, recovery of these silicate materials by skimming apparently was not easily accomplished.
One prior art attempt at to develop a floating wick type of device is seen in U.S. Pat. No. 3,667,982 to Marx. Marx discloses a “cellulose sponge” formed of an artificial or purified cellulose material created through a conventional viscose process. This purified cellulose eliminates many constituents found in natural cellulose such as lignins, hemicelluloses, waxes, oils and other components functioning to confine water to vascular tissues and to inhibit water uptake by cellulose's structural components. Marx then treats his purified cellulose with an agent to render the cellulose material hydrophobic and oleophilic. The agent is disclosed as styrene, methyl methacrylate, or furfural methacrylate in combination with an organic peroxide such as benzoyl peroxide or methyl ethyl ketone peroxide.
However, it has been discovered that the treated cellulose sponge of Marx has several significant disadvantages. For example, when sponges manufactured according to the Marx disclosure were exposed to water alone, the sponges did often remain hydrophilic. However, when the Marx sponges were exposed to oil floating on water for any significant time (e.g. more than 0.5 hours), the sponges lost their hydrophobic character and tended to become saturated with water. Additionally, Marx's sponges did not appear to consistently burn on a 1 mm continuous slick for more than 45 minutes and failed to burn most of the oil absorbed. Apparently the styrene/benzoyl peroxide treating agent is soluble in hydrocarbons and loses its ability to retard water uptake in a relatively short time after ignition of the sponge. Consequently, water saturation quickly limits the sponge's ability to remain ignited and efficiently burn oil on the water surface.
Another attempt at floating devices designed to facilitate burning of oil on water is illustrated in U.S. Pat. No. 4,154,684 to Tokarz. The Tokarz patent discloses a small disk constructed of a large number of hollow ceramic “microspheres” bonded together with an epoxy resin. In addition to being significantly more expensive than cellulosic materials, the ceramic disks of Tokarz are not subject to natural decay and remain buoyant, and therefore must be recovered to prevent eventual shoreline contamination. Moreover, the Tokarz disks are not digestible and unrecovered floating disks may prove a hazard to marine life. Thus, any unrecovered Tokarz disks potentially pose a significant environmental hazard.
What is needed in the art is a small, floating device to concentrate and support burning of thin films of floating oil, but which is readily recoverable and is not a danger to marine life forms. Such a floating device should be capable of overcoming the disadvantages found in devices such as disclosed in Marx and Tokarz.